The adhesive bonding of metal parts to plastics is typically effected using double-sided pressure-sensitive adhesive tapes. The adhesive forces required for this purpose are enough to attach and fasten the metal components to the plastics. Metals used are typically steel, including stainless steel, and aluminum. Plastics used are, for example, PVC, ABS, PC or blends based on these plastics. For portable consumer electronics articles, however, the requirements are continually rising. On the one hand these articles are becoming ever smaller, and so the bond areas too automatically become smaller. On the other hand, the bond must also meet additional requirements, since portable articles are employed across a very large temperature range and, moreover, may be exposed to dropping. These requirements are particularly problematic for metal bonds to plastics. In the event of a drop, the plastic may absorb some of the energy, whereas metals do not deform at all. In this case the adhesive tape has to absorb a large part of the energy. This can be done particularly efficiently through the use of heat-activatable sheets, which are able to develop a particularly high adhesive force following activation. For mediation of adhesive qualities (for the formation of an adhesive layer), heat-activatable adhesive sheets require the application of heat and in general—as in the case of all pressure-sensitive adhesives or self-adhesive compositions that are sensitive to pressure—require a certain application of pressure (and should not, therefore, be confused with thermosetting reactive adhesives, which cure when heat is supplied and cannot therefore be considered self-adhesive compositions).
Heat-activatable adhesives can be divided into two categories:    a) thermoplastic heat-activatable sheets    b) reactive heat-activatable sheets.
Thermoplastic heat-activatable sheets have already been known for a long time and are based, for example, on polyesters or copolyamides. Commercial examples thereof are 3M 615, 3M 615S or tesa 8440. For application in portable consumer electronics articles, however, these thermoplastic heat-activatable sheets also have disadvantages. This relates in particular to the oozing behavior under pressure and temperature application, since diecuts are always processed in the application, and then alter their shape.
It is also possible to use reactive heat-activatable sheets. These possess significantly better dimensional stability if the elastomeric component has a high elasticity. Moreover, the reactive resins allow a crosslinking reaction to occur that significantly increases the bond strength. For this bonding, accordingly, it is possible to use, for example, heat-activatable sheets based on nitrile rubbers and phenolic resins, as available commercially, for example, through tesa 8401. A disadvantage of these reactive heat-activatable sheets, however, is the dependence of the bond strength on the curing conditions. Particularly exacting requirements are imposed here, since consumer electronics devices are manufactured in massive numbers and hence the individual components are produced in very short cycle times.
The high flow viscosity of the nitrile rubber gives the heat-activatable sheet a high dimensional stability and, as a result of the crosslinking reaction, allows high adhesive forces on metals and plastics. The high dimensional stability and low flow capacity, however, also possess disadvantages: as a result of the high strength, the heat-activatable sheet hardens very quickly at low temperatures and becomes brittle, with the result that at very low temperatures the bond becomes shock-sensitive and cracks.
Success has not hitherto been achieved, unfortunately, in producing a heat-activatable sheet in a form such that the bond strength is very high at both high and low temperatures and hence allows a very wide temperature range to be covered.
In the light of this prior art, the object on which the invention is based is that of providing a heat-activatable adhesive sheet for attaching metal parts to plastics for portable consumer electronics articles which withstands a cold shock test at −20° C. and features a high bonding strength in a temperature range from −20° C. to +50° C.